A clause can contain any number of actions and variable manipulations. If a clause is left empty, the default action is taken. The default action is to trace the enabled probe identifier (EPID) to the principal buffer. For more information about epid, see Section 2.9.5, “Built-In Variables”. From the EPID, the dtrace command outputs the following information: CPU, probe ID, probe function, and probe name.

The default action facilitates a simple use of the dtrace command. For example, running the following command enables all of the probes in the vmlinux module with the default action:

# dtrace -m vmlinux

The preceding command might produce output similar to the following:

# dtrace -m vmlinux
dtrace: description 'vmlinux' matched 35 probes
CPU     ID                    FUNCTION:NAME
  0     42                 __schedule:sleep 
  0     34             dequeue_task:dequeue 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     24             enqueue_task:enqueue 
  0     41               __schedule:preempt 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     11        update_process_times:tick 
  0     42                 __schedule:sleep 
  0     34             dequeue_task:dequeue 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     24             enqueue_task:enqueue 
  0     41               __schedule:preempt 
  0     40               __schedule:off-cpu 
  0     23        finish_task_switch:on-cpu 
  0     11        update_process_times:tick 
  0     12            try_to_wake_up:wakeup 
  0     42                 __schedule:sleep 
...